The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk, and when the disk rotates, air adjacent to the disk moves along with the surface of the disk. The slider flies over the surface of the disk on a cushion of this moving air. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head traditionally includes a coil layer embedded in an insulation stack, the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic transitions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
In order to meet the ever increasing demand for improved data rate and data capacity, magnetic write heads have been constructed ever smaller. Write pole width reduction is one of the many on-going challenges in the magnetic head industry. On the one hand, deep ultra violet (DUV) photolithography processes for imaging and plating a P2 pedestal have allowed engineers to image smaller P2 pedestals, on the other hand, it provides a smaller depth of focus. As a result DUV photolithography provides less of a straight zone for the P2 pedestal.
Due to this photolithography limitation, P2 thickness (P2T) must be reduced at plating accordingly. After shaping P2 by ion mill trimming, P2T is reduced by 1.5 to 1.6 um. In a stitched pole design, aggressive ion milling processes, used to remove the coil seed layer, P3 seed layer and Cu stud seed layer, further consume P2T by 0.3 to 0.4 um. The final P2T could be only as high as 1.2 um which barely meets the P2 thickness requirement for the current devices.
In general, a long ion milling trim will degrade pole width (P2B) uniformity. A small photo patterned pole width is always preferred. This requires thinning down photo resist to improve resolution so that a small pole can be resolved without degrading in P2B variation. However, due to the current photolithography process limitations and the material removal caused by the previously discussed seed layer removal processes such as ion milling that aggressively consume P2T, further reduction in the as-plated P2T is not possible. The thickness of the photoresist can only be reduced if the consumption in P2T can also be reduced further down the line. Once the photoresist thickness is decreased, engineers can image smaller P2B and thus reduce the P2B sigma and the shape and variation within a wafer.